1. Field of the Invention
This invention relates generally to subsea anchors and methods of installing the same, and more particularly, but not by way of limitation, to such anchors utilized for anchoring an offshore oil and gas drilling and production platform.
2. Description of the Prior Art
An offshore exploration for oil and gas from subsea deposits has expanded into deeper and deeper waters, conventional rigid towers setting upon the ocean floor and extending upward to the surface have become more and more impractical.
One particular solution to this problem which has been proposed is the elimination of the rigid tower and the substitution therefor of a floating platform moored to the ocean floor by a plurality of vertical members which are placed under high tension loads due to excess buoyancy of the floating platform. Examples of such structures, which are generally referred to as tension leg platforms, are shown in U.S. Pat. No. 3,648,638 to Blenkarn and U.S. Pat. No. 3,919,957 to Ray, et al.
Typically, such a tension leg platform is designed to have a plurality of spaced clusters of vertical tension legs or tethering elements, each of said clusters including a plurality of tethers arranged in a predetermined pattern with all of the clusters additionally arranged in a predetermined pattern relative to each other, so that all of the tethers are arranged substantially vertically between the surface platform and anchors located upon the ocean floor. It is desirable that such tethers be vertically arranged, with no substantial skewing from a vertical line, because the tension variations created in the tethering elements by the forces exerted thereupon by the buoyant surface structure are greater if the tethers are non-vertical as compared to perfectly vertical.
This requires that the relative locations of all of the anchoring positions for the lower ends of the tethers be accurately located upon the ocean floor. The prior art shows several approaches to achieving this goal.
One approach is to construct a single unitary very large structure to be placed upon the ocean floor, which structure includes the anchoring points for each of the tethers and also generally includes connections for production risers or the like, which are to be connected to the floating platform. This solves the problem of providing accurate relative positioning of the tethers, but creates another problem in that the extremely large unitary anchor structure is practically difficult and expensive to manufacture, transport to the offshore drilling site, and to install at a desired location upon the ocean floor. Such structures are shown in U.S. Pat. No. 4,169,424 to Newbey, et al.; U.S. Pat. No. 3,611,734 to Mott; U.S. Pat. No. 3,648,638 to Blenkarn; U.S. Pat. No. 3,676,021 to Blenkarn, et al.; U.S. Pat. No. 3,654,886 to Silverman; and U.S. Pat. No. 4,062,313 to Stram.
Another somewhat different solution to this problem is to construct a unitary anchor structure, portions of which are hinged so as to allow the structure to partially collapse to thereby make it easier to transport, while still maintaining a predetermined spacing of the tether attachment points due to the non-variable relative positioning of those points once the structure is expanded to its final orientation. Such a structure is shown in U.S. Pat. No. 4,126,008 to Dixon.
The use of separate anchors for each cluster of tethering elements has been suggested in U.S. Pat. No. 3,919,957 to Ray, et al.; U.S. Pat. No. 3,982,492 to Steddum; and U.S. Pat. No. 3,996,755 to Kalinowski. The Steddum and Ray, et al. patents appear to disclose the same structure wherein the separate anchors are lowered to the ocean floor directly from the floating structure. The Kalinowski patent merely refers to the anchors as being preplaced without specifying any particular manner for locating and orienting the anchors.
The present invention provides a separate anchor for each cluster of tether elements and provides apparatus and methods for locating and orienting the separate anchors relative to each other and to a drilling template previously located upon the ocean floor. The anchor for each cluster of tethering elements includes a relatively lightweight and small primary anchor which is first set on the ocean floor and a larger main anchor which is subsequently lowered into engagement with the primary anchor. Means are provided for adjusting a position of the main anchor relative to the primary anchor. This is done by adjusting a relative position determining means prior to lowering the main anchor into engagement with the primary anchor. An acoustic positioning system is provided for accurately positioning the primary anchors by accurately ascertaining a lateral location and/or an angular orientation thereof relative to the previously placed drilling template. Based upon that ascertained position, the relative position determining means may be adjusted to finally locate and/or orient the main anchor at the desired position thereof relative to the drilling template and any other anchors which have previously been set. The main anchor is preferably a modular anchor comprising a plurality of interconnected modules, and each of those modules is individually attached to the ocean floor by a drilled and cemented pile.
Some of the elements of the structure of the novel apparatus of the present invention and some of the operations of the novel methods of the present invention are disclosed in the prior art.
Anchors which are attached to the ocean floor by piles driven through the anchors into the ocean floor are shown in the following references:
U.S. Pat. No. 4,039,025 to Burkhardt et al. PA1 U.S. Pat. No. 3,672,177 to Manning PA1 U.S. Pat. No. 3,572,044 to Pogonowski PA1 U.S. Pat. No. 3,648,638 to Blenkarn PA1 U.S. Pat. No. 3,976,021 to Blenkarn et al. PA1 U.S. Pat. No. 3,611,734 to Mott PA1 U.S. Pat. No. 3,654,886 to Silverman PA1 U.S. Pat. No. 3,955,521 Mott PA1 U.S. Pat. No. 3,996,755 to Kalinowski PA1 U.S. Pat. No. 4,127,005 to Osborne PA1 U.S. Pat. No. 4,129,009 to Jansz PA1 U.S. Pat. No. 4,181,453 to Vache PA1 U.S. Pat. No. 4,062,313 to Stram PA1 U.S. Pat. No. 4,126,008 to Dixon.
The use of acoustic beacons to determine location and orientation of items to be set upon the ocean floor is shown in U.S. Pat. No. 4,039,025 to Burkhardt et al., and U.S. Pat. No. 4,181,453 to Vache. Also, off the shelf systems generally adapted for such usage are available including, by way of example, a system sold by Honeywell, Inc. of Houston, Tex. designated by the trademark "RS/906" as described in Honeywell document 3333 Revision A, dated Aug. 15, 1978.
The prior art includes drilling templates which are comprised of temporary guide bases which are initially located on the ocean floor and permanent guide bases which are then lowered into engagement with the temporary guide base. An example of such structures is that sold by Vetco Offshore, Inc. as described in two of its technical bulletins entitled "Early Subsea Production Systems" and "Type SG-5 Wellhead Equipment", respectively. These Vetco systems include modular permanent guide bases which are lowered into engagement with previously positioned temporary guide bases. The anchor assemblies illustrated and described in the following disclosure are modified versions of the equipment previously sold by Vetco Offshore, Inc. for use as drilling template assemblies.
Another two part drilling template having a main template which may be lowered into engagement with a pre-placed template portion, is shown in U.S. Pat. No. 3,572,044 to Pogonowski.
Other references which generally relate to tension leg platforms or the lowering of articles from other floating structures, but which are not believed to be as relative to the present invention as the references more specifically discussed above, include U.S. Pat. No. 4,109,478 to Gracia; U.S. Pat. No. 3,943,725 to Pennock; and U.S. Pat. No. 3,986,471 to Haselton.